If there is a design flaw with the existing wiper motor, Mercedes Benz will not only have to design a new motor (or ask their supplier to design a new motor), but will have to recall all existing stock, which would explain why availability is zero.
Surely, though, if there is an issue with the wiper motor, a recall is required, or at least informing C300 owners of the issue.
I am amazed at the blatant disregard the motor industry has for its customer base, and how they try to cover things up, simply to save their profit. Vauxhall dismissed the concerns of motorists who's Zafiras had all caught fire, and did nothing until the BBC Watch Dog program highlighted this issue. I mean, how many Zafiras need to catch fire before a manufacturer admits that there is something wrong? How many people need to die before they realise all is not right?
I would suggest you contact MB UK directly and ask them to clarify why it is taking so long to replace a simple motor, a job that should not take more than an hour or two to complete. If they state that there is no UK stock, ask why. Maybe someone will come clean and say that there has been a product recall.